This invention relates to a timing belt tensioner having a one-way mechanism which allows a controlled movement of the tensioner arm or arm stops towards the tensioner free arm position.
There are several tensioner designs where the tensioner arm movements toward the free arm position are controlled with an one-way mechanism. U.S. Pat. No. 4,145,934 describes a wedge which is pushed against the arm eccentric (lever) so that the arm cannot rotate outwards once the tensioner arm is biased towards the belt by a tensioning spring. U.S. Pat. No. 4,351,636 describes a tensioner similar in principle except that the one-way wedge is replaced by a ratchet/pawl assembly. Another ratchet and pawl mechanism is described in U.S. Pat. No. 4,634,407.
Each of the above-mentioned tensioners described a one-way mechanism, which does not allow the tensioner arm to rotate away from the belt once the arm is allowed to move inwards toward the belt.
U.S. Pat. No. 4,583,962 offers an improvement to these designs by describing a mechanism which allows a limited return stroke of the arm towards the backstop required by the thermal expansion of the engine. The detail design of this patent describes a spring clutch type one-way device and an arc shape slot wherein the arm is free to rotate backwards.
U.S. Pat. No. 4,808,148 describes a design wherein the slot controlled reverse stroke is replaced by a resilient biasing element such as elastomeric spring located between the ratchet and pawl assembly and the stationary mounting member.
U.S. Pat. Nos. 4,822,322 and 4,834,694 describe tensioners wherein the one-way mechanisms are conventional one-way (roller) clutches and the arm return strokes are controlled by arc shaped slots.
U.S. Pat. No. 4,923,435 describes a tensioner wherein the arm can have a return stroke controlled by a viscous clutch between the arm and the one-way mechanism.
All of the above-mentioned known one-way devices, even those with a controlled return stroke of the arm, have a fundamental problem in allowing the one-way mechanism to move toward the belt (free arm position) whenever engine conditions are such that this can happen. In very cold engine conditions, especially those below freezing temperatures, the engine behavior can be quite erratic. Consequently, the tensioner arm can move towards the free arm position considerably more than in normally running engine conditions, either due to the engine vibrations, stiff valve train/camshaft bearings or stiff or frozen tensioner pulley bearings. This excessive arm movement results in the stops in the one-way mechanism being rotated together with the arm too far toward the free arm direction which eventually causes the tensioner arm to hit the backstop once the normal hot running conditions of the engine are reached. Since it is not normally possible to increase the amount of return stroke allowed for the arm before hitting the backstop without simultaneously increasing the possibility for the tooth skip, all of the known one-way mechanisms used to control the tensioner arm movement can create the above-mentioned failure conditions in the engines operating in cold climate conditions.
Accordingly, there exists a need for a tensioner which will solve the problems identified above.
An object of the present invention is to solve the problems enunciated above. In accordance with the principles of the present invention, this objective is obtained by providing a tensioner for an endless belt which comprises a stationary structure. A pivotal structure is mounted on the stationary part for pivotal movements in forward and return directions about a pivotal axis. A pulley is rotatably carried by the pivotal structure for rotational movement about a rotational axis parallel to the pivotal axis. A spring is provided which acts between the structures to bias the pivotal structure to pivot in the forward direction corresponding to a movement of the pulley into belt tensioning relation to a belt with which the pulley is rollingly engaged. A one-way device has structure constructed and arranged to be operable in response to the extent of pivotal movement of the pivotal structure in the forward direction to establish different positions at which pivotal movement of the pivotal structure in return direction is limited. A damping mechanism is provided having structure constructed and arranged to restrict pivotal movement of the pivotal structure in one direction as a result of sudden short-time vibratory movements or dynamic vibrations in cold conditions.